2012 GCE O Level Physics Paper 1 Suggested Solutions

冷风1985

kazuyamishima 发表于 17-11-2012 00:09
Q15 为什么是B  我觉得应该是A          压强减小 分子势能增大 所以温度必须下降 才能保证内能不变    ...

Q15
B

Actually this question is a bit difficult. Quite a few learning points can be discussed in this question.

1. Internal enery includes two parts: (a) kinetic energy of the gas particles  (b) potential energy of the gas particles.    i.e. U=KE+PE
2. Everage kinetic energy of the particles are reflected by the temperature of the gas.
    When there is no change in temperature, the kinetic energy will also remain constant.
3. When temperature remains constant, pressure and volume are inversely proportional.
4*. Potential energy is due to the distance between the particles.


In the question, the container is made of metal, which is a good conduct of heat.
When the piston is moved outwards slowly, the question actually hints that the temperature of gas is kept constant.
Approach 1:
       According to 2, there is no change in kinetic energy of the gas particles.
       When volume increases, the particles colides with the container walls less frequently, pressure reduces.
Approach 2:
       According to 3, since volume increases, pressure decreases (when temperature is kept constant).

kazuyamishima

但是体积变大 分子间的distance变大所以 potential energy会增大   问题说内能不变  温度怎么会不变？   按你说的  温度恒定  分子势能是会变大的  那样的话 内能会增大  - -

狮城留学生

kazuyamishima 发表于 18-11-2012 04:58
但是体积变大 分子间的distance变大所以 potential energy会增大   问题说内能不变  温度怎么会不变？   按 ...

我也是这么想的！

冷风1985

In O Level syllabus, we have the flowing laws:
(a) at constant temperature, p is inversely proportional to its volume.
(b) at constant pressure, p is proportional to its temperature.
(c) at constant volume, pressure is proportional to its temperature.
all the above laws are governed by the ideal gas equation pV=nRT (which is not covered in the syllabus), where n is number of moles of the gas, R=8.31 is ideal gas constant. However, for the ideal gas equation and laws to be valid, the gas is treated as ideal gas.
One of the properties of ideal gas is that: The particles have no potential energy.

Further more, potential energy has no direct relationship with pressure.